Contents
- Introduction:
- Configuration Example:
- Verification:
- Related Information:
Introduction:
In
normal routing i.e. in Unicast routing packet forwarding decisions are
typically based on the destination address of the packet arriving at a
router. The unicast routing table is organized by destination subnet and
mainly set up to forward the packet toward the destination.
In
IP multicast routing, the router forwards the packet away from the
source to make progress along the distribution tree and prevent routing
loops. The router's multicast forwarding state runs more logically by
organizing tables based on the reverse path, from the receiver back to
the root of the distribution tree. This process is known as reverse-path
forwarding (RPF).
In
short, Incoming multicast packet will not be accepted/forwarded unless
it is received on an interface that is the outgoing interface for
unicast route to the source of the packet.
Configuration Example:
In
below example multicast server S1 sends a multicast packet, with R1
flooding it to R2 and R3.R2 received its copy, and floods it as well. As
a result R3 receives the same packet from two routers:
a) On its interface fa0/0 from R1.
b) On its interface s0/0 from R2.
Topology Diagram:
Without
the RPF check, R3 would forward the packet it got from R1 to R2, and
vice versa, and begin the process of looping packets also with the same
logic, R1 and R2 also keep repeating the process. This duplication
creates multicast routing loops and generates multicast storms that
waste bandwidth and router resources.
Before
I dive into multicast configuration, let me share with you the initial
configuration of our network. All relevant configurations are below.
R1
|
R2
|
R3
|
---|---|---|
hostname R1
ip cef
!
ip multicast-routing
!
interface FastEthernet1/0
ip address 1.1.1.1 255.255.255.0
ip pim dense-mode
!
interface FastEthernet0/0
ip address 10.1.1.1 255.255.255.252
ip pim dense-mode
speed 100
full-duplex
!
interface FastEthernet0/1
ip address 10.1.1.5 255.255.255.252
ip pim dense-mode
speed 100
full-duplex
!
router eigrp 1
network 1.1.1.1 0.0.0.0
network 10.1.1.0 0.0.0.255
no auto-summary
|
hostname R2
!
ip multicast-routing
!
interface FastEthernet0/0
ip address 10.1.1.2 255.255.255.252
ip pim dense-mode
speed 100
full-duplex
!
interface Serial0/0
ip address 10.1.1.9 255.255.255.252
ip pim dense-mode
clock rate 2000000
!
router eigrp 1
network 10.1.1.0 0.0.0.255
no auto-summary
!
|
hostname R3
!
ip cef
!
ip multicast-routing
!
interface FastEthernet0/0
ip address 10.1.1.6 255.255.255.252
ip pim dense-mode
no ip route-cache
no ip mroute-cache
speed 100
full-duplex
!
interface FastEthernet0/1
ip address 3.3.3.3 255.255.255.0
ip pim dense-mode
ip igmp join-group 239.1.1.1
!
interface Serial0/0
ip address 10.1.1.10 255.255.255.252
ip pim dense-mode
no ip route-cache
no ip mroute-cache
clock rate 2000000
!
router eigrp 1
network 3.3.3.3 0.0.0.0
network 10.1.1.0 0.0.0.255
no auto-summary
!
|
When R3 performs the RPF check the following things will happen:
1) R3 examines the Source address of each incoming packet, which is 1.1.1.1.
2) R3 determines the reverse path interface based on its route used to forward packets to 1.1.1.1
In
our case R3's route to 1.1.1.1/24 is matched, and it is lists an
outgoing interface fa0/0, making fa0/0 R3's RPF interface for IP address
1.1.1.1
R3#sh ip route | beg Gate
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
D 1.1.1.0 [90/156160] via 10.1.1.5, 02:01:51, FastEthernet0/0
3.0.0.0/24 is subnetted, 1 subnets
C 3.3.3.0 is directly connected, Loopback0
10.0.0.0/30 is subnetted, 3 subnets
C 10.1.1.8 is directly connected, Serial0/0
D 10.1.1.0 [90/30720] via 10.1.1.5, 04:24:40, FastEthernet0/0
C 10.1.1.4 is directly connected, FastEthernet0/0
R3#sh ip rpf 1.1.1.1
RPF information for ? (1.1.1.1)
RPF interface: FastEthernet0/0
RPF neighbor: ? (10.1.1.5)
RPF route/mask: 1.1.1.0/24
RPF type: unicast (eigrp 1)
RPF recursion count: 0
Doing distance-preferred lookups across tables
R3#sh ip mroute | beg Interfac
Interface state: Interface, Next-Hop or VCD, State/Mode
(*, 239.1.1.1), 00:38:46/stopped, RP 0.0.0.0, flags: DCL
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Loopback0, Forward/Dense, 00:38:46/00:00:00
FastEthernet0/0, Forward/Dense, 00:38:46/00:00:00
Serial0/0, Forward/Dense, 00:38:46/00:00:00
(1.1.1.1, 239.1.1.1), 00:00:26/00:02:37, flags: LT
Incoming interface: FastEthernet0/0, RPF nbr 10.1.1.5
Outgoing interface list:
Loopback0, Forward/Dense, 00:00:26/00:00:00
Serial0/0, Prune/Dense, 00:00:26/00:02:34, A
3)
R3 compares the reverse path interface fa0/0 on which multicast packet
arrives .If they match, it accepts the packets and forward it; otherwise
,it drops the packet .In this case ,R3 floods the packets received on
fa0/0 from R1 but it ignore the packets received on s0/0 from R2.
Verification:
1)
To verify we will be sending ICMP echo to group 239.1.1.1 from R1 with
source 1.1.1.1.It's always safe to collect debugging logs in buffer
rather than on console hence we will be debugging multicast packet and
collect it in logging buffer as shown below:
R3#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#logging console informational
R3(config)#logging buffer 7
R3(config)#logging buffer 64000
R3(config)#no ip cef
R3(config)#end
*Mar 1 04:44:41.670: %SYS-5-CONFIG_I: Configured from console by console
R3#debug ip mpacket
IP multicast packets debugging is on
R1#ping 239.1.1.1 source 1.1.1.1
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 239.1.1.1, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1
Reply to request 0 from 10.1.1.6, 24 ms
Reply to request 0 from 10.1.1.6, 128 ms
R3#sh logging | beg Log
Log Buffer (64000 bytes):
IP(0): s=10.1.1.5 (FastEthernet0/0) d=239.1.1.1 (Serial0/0) id=19, ttl=254, prot=1, len=100(100), mforward
IP(0): s=10.1.1.1 (Serial0/0) d=239.1.1.1 id=19, ttl=253, prot=1, len=104(100), not RPF interface
IP(0): s=10.1.1.5 (FastEthernet0/0) d=239.1.1.1 (Serial0/0) id=20, ttl=254, prot=1, len=100(100), mforward
IP(0): s=10.1.1.1 (Serial0/0) d=239.1.1.1 id=20, ttl=253, prot=1, len=104(100), not RPF interface
IP(0): s=1.1.1.1 (FastEthernet0/0) d=239.1.1.1 (Serial0/0) id=20, ttl=253, prot=1, len=100(100), mforward
IP(0): s=1.1.1.1 (Serial0/0) d=239.1.1.1 id=20, ttl=252, prot=1, len=104(100), not RPF interface
From
the above logs we can see that R3 forwarded the packets received on
fa0/0 from R1 but it ignore the packets received on s0/0 from R2.
2) Let’s look it same with mtrace from R1 and capturing packet with the help wireshark on R3’s interface Fa0/0 and S0/0.
R1#mtrace 1.1.1.1 3.3.3.3 239.1.1.1
Type escape sequence to abort.
Mtrace from 1.1.1.1 to 3.3.3.3 via group 239.1.1.1
From source (?) to destination (?)
Querying full reverse path...
0 3.3.3.3
-1 10.1.1.6 PIM [1.1.1.0/24]
-2 10.1.1.5 PIM [1.1.1.0/24]
-3 1.1.1.1
On R3’s interface fa0/0 we capture trace route query and request as mark in black box below diagram:
Let's open the traceroute request packet to get more detail inside view.
As show in above figure “FORWORDING CODE: NO_ERROR” field
shows that after the router receives a multicast packet it performed an
RPF check as the RPF check succeeds, the packet is forwarded.
Now let’s view capture taken on interface S0/0:
It is only showing trace route query not request as packets are drop due to RPF check failure.
Hence
conclusion is the RPF check is a strategy by which router accept
packets that arrives over the shortest path and discard those that
arrive over longer routes and thereby avoid routing loops and
duplication.
No comments:
Post a Comment